Vinylidene fluoride resin is excellent in chemical resistance, heat resistance and mechanical strength and, therefore, has been studied with respect to application thereof to porous membranes for separation. Many proposals have been made regarding porous membranes of vinylidene fluoride resin, for water (filtration) treatment, particularly for production of potable water or sewage treatment, and also processes for production thereof (e.g., Patent documents 1-6 listed below).
For example, Patent documents 1 and 2 disclose process for producing porous membrane by the non-solvent-inducing phase separation method wherein a vinylidene-fluoride-resin solution is caused to contact a liquid which solidifies a vinylidene-fluoride-resin solution (generally a non-solvent for a vinylidene fluoride resin), to be solidified. However, according to this process, the diffusion (mass transfer) though a membrane of the non-solvent becomes slow and tends to result in a film on the membrane surface, thus resulting in a porous membrane with a high resin concentration (that is, a low porosity) on the treated water-side surface. On the other hand, Patent document 3 discloses a process of forming a porous membrane, wherein a relatively large amount of an organic liquid of non-mutual solubility with a vinylidene fluoride resin, such as dioctyl phthalate and dibutyl phthalate, is dispersed together with silica powder, in the vinylidene fluoride resin, and these organic liquid and silica powder removed by extraction, after the shaping.
Also, the present inventors, et al., have found that a process of melt-extruding a vinylidene fluoride resin having a specific molecular weight characteristic together with a plasticizer and a good solvent for the vinylidene fluoride resin into a hollow fiber-form and then removing the plasticizer by extraction to render the hollow fiber porous is effective for formation of a porous membrane of vinylidene fluoride resin having minute pores of appropriate size and distribution and also excellent in mechanical strength, and have made a series of proposals (Patent documents 7-10 and others). However, a strong demand exists for further improvements of overall performances including filtration performances and mechanical performances of the porous membrane necessary for use as a filtration membrane. For example, as an MF (microfiltration) membrane used for the purpose of, e.g., production of potable water or industrial water by clarification of river water, etc., or clarification of sewage, it is required to have an average pore size of at most 0.25 μm for secure removal of Cryptosporidium, Escherichia coli, etc., as typical injurious micro-organisms, and causes little contamination (clogging) with organic substances on the occasion of continuous filtration operation of cloudy water, to maintain a high water permeation rate. From this viewpoint, a porous membrane proposed by Patent document 6 below has an excessively large average pore size, and a hollow-fiber porous membrane proposed by Patent document 8 retains a problem in maintenance of a water permeation rate in continuous filtration operation of cloudy water.
Such an increase in water filtration pressure due to clogging has been considered to be an inevitable problem accompanying a water filtration operation performed for removing dispersed minute particles in water to be treated by using a hydrophobic vinylidene fluoride resin porous membrane. Of course, although trials for reducing the increase in water filtration pressure due to clogging by hydrophilization of the vinylidene fluoride resin itself or the partial hydrophilization of porous membranes formed of vinylidene fluoride resin have been conducted, an essential solution has not been attained thereby, mostly due to insufficient sustainability of the effect, or confronting problems, such as lowering in membrane strengths or resistance to chemicals.